cpu: Add a helper function cpu_get_lapic_addr

This change adds a helper function cpu_get_lapic_addr() that returns
LOCAL_APIC_ADDR for x86. It also adds a weak default implementation
which returns 0 if platform does not support LAPIC. This is being
done in preparation to move all ACPI table support in coreboot out of
arch/x86.

BUG=b:155428745

Change-Id: I4d9c50ee46804164712aaa22be1b434f800871ec
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40929
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/arch/x86/cpu.c

@@ -354,3 +354,8 @@ int cpu_index(void)
 	}
 	return -1;
 }
+
+uintptr_t cpu_get_lapic_addr(void)
+{
+	return LOCAL_APIC_ADDR;
+}